112 Mr. W. Swainson on Iiidma, 



Such appears to me to be the most natural development of 

 the confused and pai-tly contradictory accounts, which the an- 

 cients have bequeathed to us, of their knowledge and acquisi- 

 tion of tin. 



XXVI. RemarJcs on Iridina, a Gernis of Fresh-'watei- Bivalve 



Shells ; '■iSoith the Spedjic Characters of three Species. By 



William Swainson, Esq. F.li.S. F.L.S. 4'c.* 

 TyrEITHER Linnaeus nor any of his followers appear to 

 ■^^ have been acquainted with the shell, on which Lamarck 

 has founded the genus Iridina ,- a fresh-water bivalve of great 

 rarity, and supposed to be a native of South America. Only 

 one species being known to that celebrated naturalist, his ac- 

 count of it is unaccompanied by any specific character ; an- 

 other has been figured by Mr. Sowerby, under the name of 

 /. elongata, but without any definition. Having recently met 

 with a third species, I now beg to offer such specific characters 

 for all three, as may serve to distinguish them in the system; 

 subjoining at the same time a modification of the generic cha- 

 racters as defined by Lamarck. 



I. Iridina (Familia Naiada:). 



Testa aeiquivalvis, intequilatera, transversa; umbonibus decor- 

 ticatis. Cardo longus, linearis, per longitudinem tuberculosus; 

 «ubcrenatus: tuberculis inaequalibus crebris; ligamentum ex- 

 ternum, marginale. 



Shell equivalve, the sides unequal, transverse; mnbones de- 

 corticated. Hinge-plate long, linear, broken into obtuse, 

 crenated, unequal, crowded tubercles. Ligament external and 

 marginal. 



Obs. — The animal of this genus is unknown ; but, judging 

 from the shell, its situation in the system will be intermediate 

 between Unio and Anodon : the brilliancy of the pearly in- 

 terior of the valves is highly beautiful. 



Species. — 1. Iridina striata. 



I. testa transversim oblonga, antice lata, extremitate utra- 

 que striis radiatis ornata ; margine basali sinuato. 



Shell ti'ansversely oblong, anteriorly broad, with radiated 

 striae at each extremity ; basal margin sinuated. Iridina ex- 

 otica. Lam. St/st. vol. vi. p. 89. Fncy. Meth. pi. 204. {his.) 

 f. 1. a.b. 



This grand and excessively rare shell, I only know fi-om 

 the description of Lamarck, and the figure above quoted; 

 neither am I aware that it exists in any collection in this coun- 



♦ Communicated by the Author. 



try.. 



